Conventionally, there is such a system as proposed for detecting variations of resonant frequency and Q value of a cavity resonator when a sheet-like material is inserted therein, in order to measure complex dielectric constant of the material by applying microwaves. However, this system is not suited for measuring the electric conductivity of sheet-like conductive material containing carbon fiber. When the conductive material is inserted in a cavity resonator systems, the resonant condition is destroyed, and thus, even if a driving power input were sufficiently large, for the system it is extremely difficult to quantitatively detect the difference of conductivity between materials. On the other hand, carbon fiber is distinctly useful for reinforcing plastic material. Actually, carbon-fiber-reinforced plastic material is widely used today. Reflecting this, those who are concerned in this field look for a simplified method for comparatively measuring the ratio of content and degree of orientation of carbon fiber in the plastic material. Measurement of these can be achieved by applying mechanical means which first cuts off specimen having the predetermined size from the product material in various directions before carrying out tension tests. Nevertheless, measurement of those factors mentioned above requires many processes and much time and labor. Furthermore, when comparatively evaluating the conductivity between materials to satisfy the needs for applying the conductivity of material like in the case, for example of shielding the electromagnetic field, the test method mentioned above is totally useless, since the conductivity of material can not be detected by such method.